The+Covalent+Bond

Atoms typically bond to achieve the stable configuration of a noble gas, generally an octet of electrons. Two atoms that both need to gain valence electrons may share them in a bond covalently, a molecule is formed. In the formation of a covalent bond, the atoms move close enough so that the repulsive forces due to like-charged particles are balanced by the attractive forces between oppositely charged particles.

Pairs of electrons involved in forming covalent bonds are called bonding pairs. When a single pair of electrons is shared, a single covalent bond results. Remaining pairs of electrons not involved in bonding are called lone pairs. When a single pair of electrons is shared, a single covalent bond results. Remaining pairs of electrons not involved in bonding are called lone pairs. The arrangement of electrons in molecules can be illustrated by Lewis Structures, which use electron-dot diagrams. In such diagrams, shared electron pairs are represented as a pair of dots or a line.

Example: Drawing a Lewis Structure for a Molecule What is the Lewis Structure for a molecule of the covalently bonded compound nitrogen triiodide (NI3), given that all the bonds are single? State how many single covalent bonds there are and how many lone pairs each bonded atom has.

As a 5A element, nitrogen has five valence electrons and therefore requires three more electrons to achieve a complete octet. It must form three covalent bonds to do so. Iodine, a group 7A element, has seven valence electrons and needs one more electron to achieve a complete octet. It must form only one bond to do so. There are three single covalent bonds in the molecule. There is one long pair on the nitrogen atom and three lone pairs on each iodine atom.

Single bonds are always sigma bonds, bonds in which the electron pair is shared in an area centered between the two atoms. Such bonds can form when the bonding orbital is created by overlap of two s orbitals, an s and a p orbital, or two p orbitals.
 * Bond Types and Multiple Bonds**

In some cases, atoms attain a noble gas configuration by sharing more that one pair of electrons, forming a multiple covalent bond. In a double covalent bond, three pairs are shared. A multiple covalent bond always consists of a sigma bond and at leas one pi bond, a bond in which parallel orbitals overlap. A pi bond occupies space above and below the line that represents where the two atoms are joined.

Example: Identifying Bond Types in a Molecule The oxygen molecule has the formula O2. In this molecule, a total of four electrons are shared by the two atoms. What type of bond is this: single, double, or triple? Is the bond made up of sigma bonds, pi bonds, or both?

The four shared electrons make up two bonding pairs, which means that the bond is a double bond. In any multiple bond, there is one sigma bond and the remaining bonds are pi bonds. Thus, in this case, there are one sigma bond and one pi bond.

The distance between the nuclei of two bonded atoms is called bond length. When the balance of forces between the atoms is sufficiently upset, the bond can break. Energy is absorbed when a bond breaks and is released when a bond forms. The amount of energy required to break a covalent bond is called the bond dissociation energy than does a longer bond. Triple bonds are usually shorter and have greater bond dissociation energies than do double bonds. Double bonds are usually shorter and have greater bond dissociation energies than do single bonds.
 * Bond Length and Strength**

The total energy change in a chemical reaction is determined from the energy of the bonds broken and formed. In an endothermic reaction, more energy is required to break existing bonds in the reactants than is released when the new product bonds form. In an exothermic reaction, more energy is released in the formation of the new product bonds than is required to break bonds in the reactants.